This invention relates to an electrode. More specifically, this invention relates to a high-temperature-resistant electrode for use as a current collector in the channel of a magnetohydrodynamic generator.
In a magnetohydrodynamic power generator, heat is utilized to produce a high-velocity stream of electrically conducting fluid or plasma which is passed through a magnetic field to convert the kinetic energy of the stream into electrical energy. A typical diagonal window frame MHD power generator is an elongated duct or channel constructed of a large number of open rectangular forms or "window frames" fastened together side by side, insulated from each other and cooled by a liquid passing through coolant channels in each frame. Around the inner perimeter of each frame are attached, generally by brazing, a number of individual, generally rectangular, electrodes for collecting the electrical energy generated in each frame by the passage of the high-temperature conductive fluid through the magnetic field. Other generator geometries can also be used, but in each case a number of electrodes are present and separated from each other by an electrical insulation since some will act as anodes and some as cathodes as the plasma passes through the channel perpendicular to the longitudinal axis of the electrodes.
The environmental conditions within an operating channel in which the electrodes must function are very severe, and strenuous physical demands are placed on these electrodes. The plasma, which may be either ionized gas or an inert gas seeded with a conductor such as potassium, may reach temperatures up to 2800.degree. C, while the surface of the electrodes may reach about 2000.degree. C. However, since the window frames to which the electrodes are attached are generally of copper, the electrode-frame temperatures can be no more than about 600.degree.-1000.degree. C. Thus, the electrodes must be capable of withstanding a temperature differential between electrodeplasma interface and the electrode-frame interface of up to about 1400.degree. C. It is important that the thermal conductivity of the electrodes be controlled so that the heat loss from the plasma through the electrodes to the frames is not too great. Minimizing the temperature differential within the plasma between the plasma core and the electrodeplasma interface increases the energy conversion efficiency. The electrode must be able to withstand erosive forces since the plasma as it passes through the duct may approach or even exceed sonic velocity. The electrode must either be protected from oxidation or be prepared of oxidation-resistant materials since many plasmas, depending upon the particular fluid and its source, are slightly oxidizing at operating temperatures. Finally, due to the possibility of generator malfunction, the electrodes must be able to withstand the thermal shock of sudden heating or cooling without the electrode separating from the channel or without the upper high-temperature erosion-resistant layers spalling from the remainder of the electrode. Thus it is a problem to find a material or materials from which electrodes can be made which can withstand the rigors of such an environment. U.S. patent application Ser. No. 745,942, filed November 29, 1976 and assigned to the common assignee, describes an MHD electrode and electrode system which meets many but not all of the hereinbefore-enumerated problems.